Method of producing high-grade vitreous silica



Patented Get. ll, I927.

uurrao STATESPATENT ounce.

HAROLD L WATSON, 0F LYNH, MASSACEHSETTB, ASSIGN OR T0 GENERAL EHAEQEBIGI A COBPOBATIGN OF NEW YGRK.

*HETHGD OF PRODUCING HIGH-GRADE VITBEOU$ SILIQE.

Bypassing,

The present invention relates to the pro- ;is very sluggish or viscous,the bubbles do not escape. The Word fusion is used here to designate asufiicient softening of the silica to permit it to flow. This occurs atabout 1*?00" to 18 96" C. At higher temperatures the fusion becomes morenearly liquid but as volatilizatiou occurs as the tempera ture israised, the removal of the bubbles in fused silica has constituted adifficult prob lem.

In Patent No. 1,549,597 issued to Levi B. Miller on August 11, 1926, amethod of producing bubble-free silica 1S descrlbed whereby crystallinesilica first is heated to fusion in a good vacuum and the vacuum-fusionproduct then is re-fused in an ordinary furnace open to the air Wherethe fusion is subjected to pressure. mechanically applied. Fusion in avacuum prevents substantially v the trapping of gas in substantialamounts within the silica when the crystalline ma "rielshatters andfuses, and the subsequent pressure step further reduces the size andnumber of the bubbles or cavities.

I have provided an improved method of clarifying silica, which in itsspecial application is used in place of the pressure fusion to furtherrefine vacuum-fused silica, and which also may be used beneficially forfusing silica not fused reviously in a vacuum. In accordance with myinvention silica is heated to a temperature materially higher than thetemperature at which silica fuses, preferably to a temperature wellabove 2,000 C. until trapped gases are eliminated therefrom.

In carrying out my invention in its preferred form, vacuum-fused silicasuch as produced by the method described and a vacuum furnace shown inDevcrs United States Patent No. 1,536,821, is packed in a suitablecontainer, for example, in a graphite crucible, and heated to atemperature of a temperature above 2000 C. in the application filed time24, 1826. Serial 30.11839l.

about 2000 to 2300 C. for a short time,-eay for about ten minutes. Thesilica preferably 1s surroundcdb' a paclrin material of loosefinegraphite a; ich also 5 ould be preheated to 2000" C. to remove imurlties. A resist-I ance rurnace may be emp eyed for this pur pose.v Forexample, I may use the furnace shown in Thomson and Wateon'Patent bio.1,546,266 of July 14,1925, or'in the furnace described inMillerPatcntNo. 1,549,597-0f August .11, 1925. In some casesit is prefer able to,use the ordinary resistance furnace,

packed in charcoal. Although the vaporizatron of silica occurs at anaccelerated. rate when the temperature is raised above the in which theheater consists cl 9. carboutube i are because of the shortness of thetime intervals required in the practice of ray invention to eliminatebubbles. At a temperature oil about 2300 G- vacuum-fused silicasubstantiully free from bubbles is produced with a loss of about 30 percent. The fusion prefe. ably is carried out under atmospheric pressurealthough fusion in a vacuum at the high temperature characteristic of myinvention I is permissible.

In my opinion the removal of the bubble in the silica is due to theenhanced absorptive property of vitreous silica for gas at hightemperature. In silica pro-fused in vacuo very little gas is actuallytrap and in all probability the bubbles, of w 'ch the larger are aboutone millimeter in diameter,

contain highly attenuated gas. The absorgtion of this gas and the lowerviscosity of silica in the superheated state causes a sub stantialelimination of the bubbles: In addition to the elimination'of gas byabsorption, gas also escapes from the fused silica because of thegreater fluidity of silica at temperatures materially above 1800 G,particularly so as thegas expandsa's the temperature is raised.

What I claim as new and desire to secure" silica containing trapped gas'w'wmh oonsists in heating said silica to a tempeyature of about 2300 C.for a short time 3. The process of prociucing clear, vit- 5 reous silicawhich consists in heating crystalline silica in a vacuum to atemperature suificiently high only to cause fuslon and HARGLD L. WATSON.

